Boeing recently launched a new line of aircraft, the 787 Dreamliner, that they claim uses 20% less fuel than existing, similarly sized planes.

How did they pull off this sizeable bump in fuel efficiency? And can you always build a more fuel-efficient aircraft? Imagine a hypothetical news story, where a rival company came up with a new type of airplane that used half the fuel of its current day counterparts. Should you believe their claim?

More generally, do the laws of physics impose any limits on the efficiency of flight? The answer, it turns out, is yes.

There’s something about flying that doesn’t sit well with us. If we never saw a bird fly, it may never have occurred to us to build flying machines of our own.

Here’s where I think this sense of unease comes from. It takes stuff to support stuff. Everyday objects fall unless other things get in their way. Take the floor away, and you’ll plummet to your doom – the air below your feet isn’t going to do much for you. We move through air so effortlessly, that we barely notice it’s there. So what keeps a plane up? There doesn’t seem to be enough ‘stuff’ there to hold up a bird, let alone a Boeing aircraft weighing up to 500,000 pounds. To put that last number in context, its more than the weight of an adult blue whale!

Why is it that planes fly and whales typically don’t? The answer is easy to state, but its consequences are rather surprising. Planes fly by throwing air down. That’s basically it. It’s an important point, so I’ll say it again. Planes fly by throwing air down.

As a plane hurtles through the air, it carves out a tube of air, much of which is deflected downwards by the wings. Throw down enough air fast enough, and you can stay afloat, just as the downwards thrust of a rocket pushes it up. The key is that you have to throw down a lot of air (like a glider or an albatross), or throw it down really fast (like a helicopter or a hummingbird).

A physicist’s two-step guide to flight (it’s simple, really!)

Let’s make this idea more quantitative. Following David MacKay’s wonderful book on Sustainable Energy, I’m going to build a toy model of flight. A good model should give you a lot of bang for the buck. The means being able to predict relevant quantities about the real world while making a minimum of assumptions.

Step 1: Sweep out a tube of air

As a plane moves, it carves out a tube of air. This air was stationary, minding its own business, until the airplane rammed into it. This costs energy, for the same reason your car’s fuel efficiency drops when you speed up on the highway. Your car has to shove air out of its way.

Exactly how much energy does this cost? You might remember from high school physics that it takes an amount of energy equal to to bring stuff with mass up to a speed .

In our case, we have

There’s still this mysterious factor of the mass of the air tube. To work this out, we can use a favorite trick in the toolbox of a physicist – unit cancellation. We can re-write the humble kilogram as a seemingly complicated product of terms.

What we’ve done here is to express an unknown mass of air in terms of other quantities that we do know. Each of these terms makes sense. Air that’s more dense will weigh more. A fatter plane (larger cross-sectional area) sweeps out more air, as does a faster plane. We’ve arrived at a meaningful result, just by playing around with units. In the words of Randall Munroe, unit cancellation is weird.

Put these two ideas together and here’s what you find:

Here’s a graph of what that looks like.

If you’re with me so far, we just found that for a plane to plow through air, it has to expend an amount of energy proportional to the speed of the plane to third power. (The extra factor of v comes from the fact that faster planes sweep out a larger mass of air.) If you want to go twice as fast, you need to work 8 times as hard to shove air out of your way.

We’ve arrived at a general rule about the physics of drag. This holds true for a car on the highway, or for a swimmer or cyclist in a race. It’s why drag racing cars get only about 0.05 miles to a gallon! If we want to reduce overall energy consumption by cars, one option is to lower the speed limits on highways.

What does this mean for our toy plane? It would seem that the slower the plane, the higher its efficiency. So are airplane speed limits also in order? Absolutely not! To see why, read on to the second half the story..

Step 2: Throw the air down

In order to fly, a plane must throw air downwards. This generates the lift that a plane needs to stay up. It turns out that slower planes have to throw air harder to stay afloat. That’s why slow moving hummingbirds and pigeons have to flap their wings frenetically. It’s also why planes extend flaps while landing – they’re not throwing the air fast enough, so they compensate by throwing more of it.

More precisely, for a plane to stay afloat, the speed of the air jettisoned downwards must be inversely proportional to the speed of the plane. (You can take my word for this, although if you want to see where it comes from, take a look at David MacKay’s book.)

So we can now work out the second part of the puzzle. How much energy does it take to throw air down? As before, this is given by

Just as we did in the first step, let’s express things in terms of the speed of the plane.

In words, the energy spent in generating lift is inversely proportional to the speed of the plane. Here’s what this looks like on a graph.

You can see from the plot that, as far as lift is concerned, slower flight is less efficient than faster flight, because you have to work harder in throwing air downwards.

There’s a lot to chew on here. To summarize, we’ve discovered that in making a machine fly, you have to spend energy (really fuel) in two ways.

1. Drag: You need to spend fuel to push air away. This keeps you from slowing down.
2. Lift: You need to spend fuel to throw air down. This is what keeps the plane afloat.

The total fuel consumption is the sum of these two parts.

If you fly too fast, you’ll spend too much fuel on drag (think of a drag racer or an F-16). Fly too slow, and you’ll have to spend too much fuel on generating lift, like a hummingbird furiously flapping its wings, powered by high calorie nectar. However, at the bottom of this curve there is a happy minimum, an ideal speed that resolves this tradeoff. This is the speed at which a plane is most efficient with its fuel. Be it through the ingenuity of aircraft engineers, or the ruthless efficiency of natural selection,  airplanes and birds are often fine-tuned to be as energy efficient as possible.

Here’s a plot of experimental data of the power consumption of different birds, as their flight speed varies.

You can see that it matches the qualitative predictions of the toy model.

But we can do more than this, and actually extract quantitative predictions from the model. An undergraduate schooled in calculus should be able to work out that special optimal speed at which energy consumption is a minimum. David MacKay plugs in the numbers in  his book, and finds that the optimal speed of an albatross is about 32 mph, and for a Boeing 747 is about 540 mph. Both these numbers are remarkably close to the real values. Albatrosses fly at about 30-55 mph, and the cruise speed of a Boeing 747 is about 567 mph. 

That’s a lot of mileage from a toy model!

And with this physicsy interlude into the world of albatrosses, hummingbirds, and jet planes, we come back to the question of the fuel efficiency of Boeing’s new aircraft.

You can actually use the model to work out the fuel efficiency of a plane. What you find is that it really just depends on a few factors: the shape and surface of the plane, and the efficiency of its engine. And of these factors, the engine efficiency plays the biggest role. So we would predict that engine efficiency, followed by improvements in body design might drive Boeing’s fuel savings.

This agrees with Boeing’s own assessment.

New engines from General Electric and Rolls-Royce are used on the 787. Advances in engine technology are the biggest contributor to overall fuel efficiency improvements.

New technologies and processes have been developed to help Boeing and its supplier partners achieve the efficiency gains. For example, manufacturing a one-piece fuselage section has eliminated 1,500 aluminum sheets and 40,000 – 50,000 fasteners.

Try as we like, we can’t squeeze a lot of improvement out of airplanes. Engines are already remarkably efficient, and you certainly can’t shrink the size of a plane by much, as economy class passengers can well attest. New manufacturing techniques could cut the amount of drag on the plane’s surface, but these improvements would only raise fuel efficiency by about 10%.

To quote David Mackay,

The only way to make a plane consume fuel more efficiently is to put it on the ground and stop it. Planes have been fantastically optimized, and there is no prospect of significant improvements in plane efficiency.

A 10% improvement? Yes, possible. A doubling of efficiency? I’d eat my complimentary socks.

References

I based this blog post on material I learnt from David MacKay’s fantastically clear book, Sustainable Energy without the Hot Air. It’s available online for free, and is highly recommended for anybody looking to use numbers to understand energy.

David MacKay (2009). Sustainable Energy – Without the Hot Air UIT Cambridge Ltd

I used this tip to make those XKCD style plots.

The Maasai and their Diet

The Maasai are a pastoralist tribe living in Kenya and Northern Tanzania. Their traditional diet consists almost entirely of milk, meat, and blood. Two thirds of their calories come from fat, and they consume 600 – 2000 mg of cholesterol  a day. To put that number in perspective, the American Heart Association recommends consuming under 300 mg of cholesterol a day. In spite of a high fat, high cholesterol diet, the Maasai have low rates of diseases typically associated with such diets. They tend to have low blood pressure, their overall cholesterol levels are low, they have low incidences of cholesterol gallstones, as well as low rates of coronary artery diseases such as atherosclerosis.

Even more remarkable are the results of a 1971 study by Taylor and Ho. Two groups of Maasai were fed a controlled diet for 8 weeks. One group – the control group – was given food rich in calories. The other group had the same diet, but with an additional 2 grams of cholesterol per day. Both diets contained small amounts of a radioactive tracer (carbon 14). (You’d never get approval for a study like this today, and for good reason.) By monitoring blood and fecal samples, the scientists discovered that the two groups had basically identical levels of total cholesterol in their blood. In spite of consuming a large dose of cholesterol, these individuals had the same cholesterol levels as the control group.

Here is how the authors concluded their study:

This led us to believe, but without direct proof, that the Masai have some basically different genetic traits that result in their having superior biologic mechanisms for protection from hypercholesteremia

Motivated by these results, we set out to identify genes under selection in the Maasai as a result of these unusual dietary pressures. We scanned the genome looking for genetic signatures of natural selection at work.

The Data

Our data comes from the International HapMap Project, a collaborative experimental effort to study the genetic diversity in humans. The HapMap Project has collected DNA from groups of people from genetically diverse human populations with ancestry in Africa, Asia and Europe. Their anonymized data is publicly available for free. One of the HapMap populations is a group of Maasai from Kinyawa, Kenya  (n=156), and this is the population that we focus on.

HapMap does not sequence full genomes, as this would have been prohibitively expensive at the time of data collection. Instead, they employ a shortcut. If you take my DNA sequence and line it up against yours, the two sequences will be about 99.9% similar. But every once in a thousand letters, or so, there will be a difference. You may have an A where I have a C. The HapMap group measures the DNA sequence at these very locations, where humans are known to vary from each other. In essence, they’re sampling the genome, looking only at sites where we expect to see variation. In the jargon of the field, this method is called looking for Single Nucleotide Polymorphisms, or SNPs (pronounced snips).

Hunting for signatures of selection in genetic data

Once you have the data, what can you do with it? We wanted to detect signs of natural selection. The basic idea behind detecting selection in genomic data is quite simple, and it has to do with sex. Every sperm or egg cell that you produce contains a single genome, which is formed by shuffling together the two sets of genomes that you inherited from your parents. Viewed this way, the role of sex is to shuffle together the genomes in a population into new combinations. If you compare the DNA sequences of a group of people, you will see signs of this shuffling.

Now lets add natural selection to the mix. What happens if an individual is born with a new mutation that benefits their survival? Over time, you’d expect to see this mutation rise in frequency. Descendants of this individual will be over-represented in the population, as the fraction of people with this beneficial mutation goes up. In essence, the fingerprint of such selection is a reduction of genomic diversity. (I’m describing a particular model of selection here, known as positive natural selection. Some other types of selection can increase diversity, such as the selection on viruses to evade recognition by their host’s immune system.)

Eventually, new mutations will creep in again, and generations of sexual reproduction would build back the diversity. However, if the loss of diversity was sudden enough (strong selection) and not too long ago, you can still detect it today. There are statistical tests (Fst, iHS, XP-EHH) that can formally detect if the reduction in diversity at a given region is sufficient to infer selection. Sabeti et al have a nice review paper that discusses the different methods available to detect selection using genomic data.

Our Results

We used three different methods to detect selection, and our top candidate regions under selection are considered significant by at least two of the methods.

The strongest signal of selection was a region on Chromosome 2 that contained the LCT gene producing lactase, the enzyme that breaks down the lactose in milk. Interestingly, the default state in all adult mammals is to stop producing lactase in adulthood – our ancestors were all ‘lactose intolerant’. This makes sense from an evolutionary point of view, it forces children to wean from milk, and frees up the mothers resources. It turns out that different sets of mutations arose that gave European and African pastoralists the ability to digest milk. Those of us whose ancestors weren’t pastoralists still have trouble digesting milk.

This is a classic example of a selective sweep – a mutation confers an advantage (the ability to digest milk), and then sweeps through a population like wildfire. This result has been previously described in European populations, and also in African populations (including the Maasai) by Sarah Tishkoff and collaborators. Given that the Maasai consume large amounts of milk, it is not surprising that we see a very strong signal at this locus. We sequenced DNA in this region to confirm this result and, sure enough, we found that one of the lactase persistence conferring mutations identified by Tishkoff was present in the HapMap Maasai samples.

Two of the tests for selection that we used require that you make comparisons with another population. We chose the Luhya of Kenya as a our reference population. Among all the protein-altering mutations present in the data, the one that showed the largest population difference between the Maasai and Luhya (as measured by Fst) sits in the gene for a fatty acid binding protein FABP1. This protein is expressed in the liver, and the variant that occurs at higher frequency in the Maasai is associated with a lowering of cholesterol levels in Northern German women (n = 826) and in French Canadian men consuming a high fat diet (n = 623). Furthermore, studies in mice fed a high fat, high cholesterol diet showed that deactivating the FABP1 protein leads to protection against obesity, and lower levels of triglycerides in the liver, when compared to normal mice on an identical diet. These results suggest that this protein plays a role in regulating lipid homeostasis, and its selection in the Maasai may be diet-related.

On Chromosome 7, two of the methods we used to detect selection identified a cluster of genes that fall in the Cytochrome P450 Subfamily 3A (CYP3A). This family of genes is involved in drug metabolism, in oxidizing fatty acids, and in synthesizing steroids from cholesterol.

What’s next?

Computational methods can only take you so far. We have identified genes in candidate regions undergoing positive natural selection in the Maasai, possibly arising due to their unusual diet. But the case for selection can only be definitively made with an experimental study targeted to address the role of these genes in maintaining cholesterol homeostasis. We’re hoping to collaborate with experimental biologists to take these hypotheses forward and investigate their role in the evolutionary history of the Maasai.

So head over to PLOS, check out the paper, and let us know what you think.

Update: Here’s another blog post that discusses the paper, focusing more on the mixed genetic makeup of the Maasai.

References:

Kshitij Wagh, Aatish Bhatia, Gabriela Alexe, Anupama Reddy, Vijay Ravikumar, Michael Seiler, Michael Boemo, Ming Yao, Lee Cronk, Asad Naqvi, Shridar Ganesan, Arnold J. Levine, Gyan Bhanot (2012). Lactase Persistence and Lipid Pathway Selection in the Maasai PLOS ONE, 7 (9) : 10.1371/journal.pone.0044751

If you’d like to read more about selective sweeps, you may enjoy my post Why moths lost their spots, and cats don’t like milk. Tales of evolution in our time.

Take the case of the mantis shrimp. These incredible crustaceans come in two varieties: stabbers, and smashers. Sheila Patek is a biologist who studies them for a living. In a fascinating TED talk from 2004, she describes how mantis shrimp have the fastest blow in the animal kingdom. Their strike force is so great that it creates a visible shock wave in water, in a bizarre phenomenon known as cavitation. Patek goes on to describe the engineering solutions that these animals use to create and sustain their powerful smash.

Since 2004, the list of nature’s fastest has had more than a few additions. It’s the time of the year for holiday lists, so I decided to list some of the most impressive record holders in this regard. To do this, I relied mainly on references I found on the wonderful website of Patek’s lab.

The life forms that follow are pushing the limits of physics and engineering. Typically, they are doing this to rein death and terror onto hapless prey. They are the Terminator 2’s of our world. So please join me, as we descend down this list towards the most lethal of all blows. This is a quest for the fastest draw in nature.

But first, let’s start with something fast that we’re familiar with. When talking about short intervals of time, we often use the phrase ‘in the blink of an eyelid’. The time it actually takes us to blink an eyelid is about 3 tenths of a second or 300 milliseconds.

A blink of an eye  (300 milliseconds)

So, our first point of reference is 10 milliseconds, or 1/30th of a blink of an eye

The ballistic tongue of the salamander (< 10 milliseconds)

The explosive tongue of the giant palm salamander of Central America bursts out in under 10 milliseconds, targeting flying bugs that don’t know what hit them. To achieve this feat, the tongue of this cold blooded sniper needs to output energy at the rate of a whopping 18,000 Watts per kilogram of muscle.

It stores this energy like a tightly coiled spring. As it relies on the principle of a slingshot, it can even operate in cold temperatures when muscles are slow to contract.

This tongue has been called the world’s most powerful muscle, but it’s no comparison to what follows.

The vacuum suction of the anglerfish (<5 milliseconds)

An anglerfish has what seems like a rather improbable fishing strategy.

It lures its prey in with a shiny dangling object attached to its head. All of a sudden, its mouth expands to more that 12 times its original size. The low pressure region thus created sucks in water at great speed, as well as whatever unfortunate fish happens to be swimming nearby. It’s a process that looks alarmingly like this.

And this strange kiss of death can take place in less than 5 milliseconds, or 1/60th of a blink of an eye.

The blinding strike of the mantis shrimp (2.7 milliseconds)

This has to be one of the most impressive punches in nature.

Sheila Patek and collaborators measured that the blow of the mantis shrimp can reach a peak speed of 51 mph (23 m/s), in less than 1/100 of the blink of an eye. All this while underwater! It’s so fast that it actually creates a visible shock wave. Meanwhile, its limb experiences over 10,000 g of acceleration.

To put this number in context, think of this: a typical person can handle an acceleration of about 5 g before losing consciousness, while decelerations of 100 g are about the highest that humans have survived, in Indy car racing accidents. A bullet shot out of a Beretta gun is accelerated by about 40,000 g.

If you were a snail or a clam, this could well be the last thing that you see:

Needless to say, a mollusk doesn’t stand much of a chance against this punch. The muscle that powers this impressive blow is delivering a mind-numbing 470,000 Watts per kilogram. It’s quite literally blowing the competition out of the water.

Well.. not quite. Read on.

The trapdoor stomach of the bladderwort plant  (2 milliseconds)

The first plant to enter our list is, of course, carnivorous, and it has an insidious method of devouring its prey. Bladderworts are a genus of over 200 carnivorous plants, all of whom capture tiny animals with a bladder-like trap. I’ve been frightened of the bladderwort ever since I watched my favorite of all David Attenborough documentaries, the Private Life of Plants, that uses time lapse footage to demonstrate their chilling strategy.

Picture this. A plant with a tiny transparent capsule. The inner walls of the capsule pump water out, and in so doing create a partial vacuum inside. The outer walls of this capsule are lined with tiny, sensitive hairs. The trap is set.

If a mosquito larvae has the misfortune of brushing against these hairs, it triggers a trapdoor that opens in 2 thousandths of a second. The insect is sucked in with an acceleration of 600 g, so escape would literally be a miracle.

The swirl of water closes the trapdoor. One set of glands then secretes juices that digest the prey, while the other set sucks the water out. In just two hours, the trap is ready to be used once again and the prey has been dissolved.

Here is a video of scientists describing this process, with some bizarrely gratuitous sci-fi sounds thrown in at appropriate moments.

By now, we’ve moved down in time by an order of magnitude, to the level of 1 millisecond. Keep in mind, 1 millisecond is 1/300th of a blink of an eye

At this point, we reach the first (and only) life form in our list that isn’t using its speed to hunt.

The pollen cannon of the bunchberry dogwood (0.3 milliseconds)

At only 2 millimeters in size, you wouldn’t normally notice this tiny flower. But this inconspicuous flower is like a loaded gun, waiting for the right conditions to go off.

When triggered, its petals unfurl with incredible force, jettisoning its pollen out in a thousandth of the blink of an eye. In this time, the pollen is accelerated by about 2,400 g, and shoots up to an inch, or about 10 times the height of the flower.

Most flowers don’t rely on wind pollination, instead opting to use insects as the distributors of their pollen. The bunchberry dogwood prefers to diversify its strategy. If a large pollinator like a bumblebee were to land on it, the pollen cannon will fire, showering the bee. However, if a smaller, less-mobile insect such as an ant were to climb onto the flower, the flower will not waste it’s precious pollen. Ants are not heavy enough to trigger the cannon. And if no insects come by, it’s not a problem, as the wind can carry the pollen a meter away.

Not bad at all, for a tiny flower.

This is not the only plant to use explosive ejaculation. At this point, I should also mention the marvelous squirting cucumber. As this cucumber shaped fruit ripens, it fills with a liquid that builds up at an immense pressure. Eventually, the pressure reaches a point where the cucumber bursts open, and the seeds shoot out with speeds over 30 miles per hour. (See the Private Life of Plants for some slow-mo action.) The squirting cucumber deserves its own entry, but I couldn’t find a reference with accurate timing information, so this is what it gets.

We’re now at a tenth of a millisecond, or 1/3000th of a blink of an eye.

The multi-purpose ballistic jaw of the trap jaw ant (0.13 milliseconds)

If you’ve ever spent some time playing a first-person-shooter computer game, you’ll know that weapons can have multiple uses. Sure, you can use that rocket-propelled grenade to attack. But you can also use it for propulsion. A well placed shot to the ground can launch you high up into the air.

The trap jaw ant understands this. Its jaws are a lethal weapon, snapping shut with an explosive force that can equal 500 times the ants weight. In a tenth of a millisecond, the jaws reach a peak speed of 143 mph (64 m/s). A quick calculation puts the acceleration of this strike in at over 50,000 g. That’s the same acceleration that a bullet experiences as it leaves a gun!

With touch sensitive hairs that serve as a trigger, and an internal latch mechanism, they can control this formidable explosive force. But what is truly incredible is how they wield it. Not only does the ant use its trap jaw for attack, it can also use it for escape.

Scientists have documented two unconventional uses of its jaw, that go by the technical names of ‘bouncer defence’ and ‘escape jump’. The latter is pretty much what it sounds like. When the ant finds itself cornered in the ant equivalent of a dark alley, it can launch itself vertically 10 cm into the air and leap to safety. The other strategy, bouncer defence, would be familiar to anyone who, like me, has wasted their childhood playing violent video games. Essentially what you do here is strike the enemy, while using the recoil to propel yourself to a safe distance.

Really, this is all just an excuse to show you this Matrix style ant video:

I’m sure you’d agree that this is a pretty sophisticated ant.

Moving down the list, we have now reached a hundredth of a  millisecond. We’re talking about a duration of time that happens 30,000 times in the blink of an eye. To put it another way, a hundredth of a millisecond is to a blink of an eye, what a blink of an eye is to two and a half hours.

Next on our list, we have:

The scissor-like jaws of the soldier termite (< 0.025 milliseconds)

The soldiers of the termite species termes panamaensis (Panama termites) are somewhat oddly shaped fellows. Their considerable, sword-like jaws and large, muscular heads take up more than half their bodies. The reason for this unwieldy headgear becomes abundantly clear when an unfriendly termite passes by. More than 70% of the time, this encounter results in death for the visitor (the number becomes 85% if you only consider visiting worker termites).

So how is this soldier termite butchering its foes with such ruthless efficiency? The key lies in speed. Its jaws snap open like a scissor, reaching a peak speed of 150 mph (67 m/s) in under 25 millionths of a second.

To power this absurd feat of strength, it relies on muscle that is delivering a peak power output of  11 Million Watts per kilogram. As far as I know, this is the most powerful muscle ever studied.

And now, we finally arrive at the end of the list, crossing a new threshold of speed. We have reached a thousandth of a millisecond, or a microsecond. This is to a blink of an eye, what a blink of an eye is to a day.

And the title of nature’s fastest draw (so far) goes to:

The retractable stingers of the jellyfish (0.0007 milliseconds, or 700 nanoseconds)

There’s fast, and then there’s mind-bogglingly, overwhelmingly, blazingly fast.

When a jellyfish detects its prey, it extends a kind of venomous vein. Like fiery filaments of doom, the job of these hair-like barbed structures is to inject neurotoxins into its prey. Here’s a video of this happening, shot under a microscope at 400x magnification.

Just how fast does a jellyfish arm itself? It turns out that the acceleration of these stingers as they emerge is 5,410,000 g. That’s not a typo.

Let me put it this way. The speed of light is a foot per nanosecond. So, in the time it takes for a jellyfish to whip out its stingers, light has travelled a distance of two football fields. It’s a timescale so fast, that the astronomical shifts down to the mundane.

And it is at this extreme scale where our journey ends, a scale where evolution is pushing up against the very laws of nature and against the speed limit of our universe. I’m excited (and a little afraid) to learn what we’ll discover next.

References:

In this article, I’ve focused on a specific way in which you can measure the fastest motion – the acceleration of an appendage relative to the body. There are, however, many other ways in which you might do this, each giving you a different champion. Here’s a splash of cold water from the lab of Dr. Patek:

Looking at peak sustained speeds – cheetahs might be the fastest. Or, focusing on peak unpowered speeds, diving falcons may be the fastest. On the other hand, if duration of the movement were the criterion of interest, nemtocysts and fungal spores would be the fastest. Lastly, considering acceleration of an appendage relative to the body through power amplification, then trap-jaw ants and termites come out on top.

Papers referenced:

Deban SM, O’Reilly JC, Dicke U, & van Leeuwen JL (2007). Extremely high-power tongue projection in plethodontid salamanders. The Journal of experimental biology, 210 (Pt 4), 655-67 PMID: 17267651

Grobecker DB, & Pietsch TW (1979). High-speed cinematographic evidence for ultrafast feeding in antennariid anglerfishes. Science (New York, N.Y.), 205 (4411), 1161-2 PMID: 17735055

Patek SN, Korff WL, & Caldwell RL (2004). Biomechanics: deadly strike mechanism of a mantis shrimp. Nature, 428 (6985), 819-20 PMID: 15103366

Vincent O, Weisskopf C, Poppinga S, Masselter T, Speck T, Joyeux M, Quilliet C, & Marmottant P (2011). Ultra-fast underwater suction traps. Proceedings. Biological sciences / The Royal Society, 278 (1720), 2909-14 PMID: 21325323

Edwards J, Whitaker D, Klionsky S, & Laskowski MJ (2005). Botany: a record-breaking pollen catapult. Nature, 435 (7039) PMID: 15889081

Patek SN, Baio JE, Fisher BL, & Suarez AV (2006). Multifunctionality and mechanical origins: ballistic jaw propulsion in trap-jaw ants. Proceedings of the National Academy of Sciences of the United States of America, 103 (34), 12787-92 PMID: 16924120

Seid MA, Scheffrahn RH, & Niven JE (2008). The rapid mandible strike of a termite soldier. Current biology : CB, 18 (22) PMID: 19036330

Nüchter T, Benoit M, Engel U, Ozbek S, & Holstein TW (2006). Nanosecond-scale kinetics of nematocyst discharge. Current biology : CB, 16 (9) PMID: 16682335

Whereas the rigid universe is notable for its strict adherence to a few basic principles, the squishy universe is a different beast altogether.

I was recently out paddling, and noticed that as you move the paddle through water, tiny whirlpools begin to develop along its sides. The whirlpools grow in size, become self-sustaining, and break off and float away. Eventually they die out, as they lose their energy to the fluid around them.

You could also watch the spirals and vortices created by rising smoke. Or notice the strange shapes made by the wind as it sweeps through the clouds. It’s as if fluids have a life of their own, often wondrous and beautiful, and other times surprising and counter-intuitive.

But the motion of fluids is notoriously hard to predict. It’s so difficult that if you can solve the equations of fluid flow, there are people willing to offer you a million dollars. The difficulty comes from a mathematical property of the equations known as non-linearity. Simply put, a non-linear system is one where a small change can lead to a large effect. The same thing that makes these equations difficult to solve is also what makes fluids surprising and interesting. It’s why the weather is so hard to predict – tiny changes in local temperatures and pressures can have a large effect.

At this point, most reasonable people would throw their arms up in despair. But physicists are an unreasonably persistent bunch, and when faced with an equation that they can’t solve, they try to get some insight by looking at what happens at extremes. For example, thick and syrupy fluids like glycerine behave in a surprisingly orderly fashion. Take a look at this video (watch through to the end, it’s worth it).

I bet you’ve never seen a fluid do that before. So what’s going on here? And what does this have to do with swimming sperm?

Let’s take a step back. Picture a flowing river. If there is an obstruction to the water’s path, like a rock jutting out of the surface, the water will move around it and swirl back upstream. Behind the rock, the water remains relatively calm. What you get is a spot on a moving river where the water is remarkably still. These calm spots are called eddies, and kayakers treat them as parking spots on the river.

But fluids don’t always behave like this. If you replace all the water in a river with a viscous fluid like glycerine, there won’t be any eddies. The syrup will simply follow the contours of the rock and smoothly flow around it.

In one case we have smooth, orderly flow, and in the other case we have eddies and turbulent flow. The question arises, is there any way to know what kind of flow will result in a given situation? This question was answered by the physicist Osborne Reynolds in 1883, and he answered it in style.

Here is Reynolds’ elegant experiment. He sent fluid flowing through a thin pipe (analogous to the river), and injected colored dye in a small section of the flow. He watched the dye flow down the tube, and could plainly see whether the flow was smooth or disorderly. By tweaking the parameters in this experiment, he was able to discover the conditions that ensure an orderly flow.

What he found is that there is one simple, magic number that can predict what is going to happen. It neatly ties together all the different physical quantities involved. It’s been named Reynolds number (Re for short), and is given by

These are all quantities that you can directly measure. The viscosity of a fluid is a measure of how slowly it flows. Thick and syrupy fluids like honey and corn syrup have a high viscosity, gases like air have a very low viscosity, and water is somewhere in between. The length in the above equation is a length that describes the object that you are studying (say the width of the rock). Reynolds used the diameter of the pipe. And the speed is that of the fluid.

The Reynolds number has the nice property of being dimensionless, meaning that the number is the same in whatever system of units you choose to measure the above quantities (dimension-full quantities are things like speed, which you could measure in km/h or mph). What Reynolds found is that as this number exceeds 2000, you suddenly get turbulent flow. In fact, this week’s issue of Science magazine mentions a new experiment that verifies this surprising result, and puts the turning point at Re = 2040. (The specifics of this number has to do with a fluid moving through a cylindrical tube with smooth walls. In a different situations, the number will change, but the principle is the same. There is a sudden jump from order to turbulence.)

The above figure gives you an idea of what happens as you increase Reynolds number. Here’s an analogy. The low Reynolds number world is like a collectivist ideal, where water moves along uniformly like soldiers marching in step. The high Reynolds number world is the individualist nightmare, where everyone looks out for themselves. Think of a march versus a mob.

We can arrive at this number from another route. There are two fundamentally different type of forces that act on an object immersed in a fluid. The first kind are inertial forces. This is like the push you give to the water when you take a stroke while swimming. Inertia is what allows water particles to keep moving undisturbed. On the other hand, you have viscous forces which measure the tendency for the fluid to smooth out any irregularities. To use the above analogy, inertial forces reflect the individuality of bits of fluid, and viscous forces are like a communist government enforcing conformity. And when you take the ratio of these forces, you get back the Reynolds number.

This number is of immense importance to aeronautical engineers and to biologists interested in locomotion.

Let’s say you want to simulate the effect of wind on a new wing design. You build a scale model in the lab that is one tenth the size of the actual wing.

But remember how the Reynolds number is defined.

If you shrink the size of the wing by a factor of 10, you have to increase the windspeed by the same amount in order to keep the number fixed. The key point is that systems with the same Reynolds number have essentially the same nature of flow. If you didn’t account for this, your wing would be quite a disaster.

How would a biologist use this idea? Well, nature presents us with organisms that cover an incredible range of sizes, from the tiniest microbes to the blue whales. Here is a table of Reynolds numbers across this range.

The list covers 14 orders of magnitude. A whale swims at a huge Reynolds number. This means that inertial forces completely dominate. If it flaps its tail once, it can coast ahead for an incredible distance. Bacteria live at the other extreme. In a delightful paper entitled Life at low Reynolds number, the physicist Edward Purcell calculated that if you a push a bacteria and then let go, it will coast for a distance equal to one tenth the diameter of a hydrogen atom before coming to a stop. And it will do this in 3 millionths of a second. Bacteria clearly inhabit a world where inertia is utterly irrelevant.

Eels and sperms may look similar, but their method of moving is very different, as their Reynolds numbers are far apart. In fact, we can now answer the question, what would it feel like to swim like a sperm or a bacteria? To do this, you have to somehow get down to their Reynolds number. We can’t change our size, but we can shrink our Reynolds number by swimming in a very viscous fluid. Purcell estimated that you would have to submerge yourself in a swimming pool full of molasses, and move your arms at the speed of the hands of a clock. (Don’t try this at home. Swimming in molasses is not a good idea.) Under these conditions, if you managed to cover a few meters in a few weeks, then you qualify as a low Reynolds number swimmer.

This clearly isn’t a hospitable environment for denizens of our Middle World. But yet this is the scale of the task that microbes face simply to get around.

Except, it’s even harder. Remember the youtube video of the colored dye swirling in the glycerine? The reason that the colors come back to where they start is because at low Reynolds number, flow is reversible. Because inertial forces are so small, certain terms drop out of the complicated fluid flow equations. The equations simplify considerably, and not only are they now solvable, they don’t depend on time any more. If you took the youtube video and played it backwards, you wouldn’t be able to tell the difference.

But this reversibility has a surprising consequence. It means that anything that swims using a repeating flapping motion can’t get anywhere. If it moves forward in one stroke, the other stroke will bring it right back to where it started. Scallops swim by opening their jaws and snapping it shut. In low Reynolds number, scallops can’t get anywhere.

Don’t believe me? See it for yourself. Here’s a rubber band powered toy that paddles forward when in water.

Woohoo! Look at it go. Now, take the same toy and place it in a vat of viscous corn syrup.

The reversibility of the flow ensures that the boat can’t make any progress.

So how, then, do microbes manage to get anywhere? Well, many don’t bother swimming at all, they just let the food drift to them. This is somewhat like a lazy cow that waits for the grass under its mouth to to grow back. But many microbes do swim, and they make use of remarkable adaptations to get around in an environment that is entirely alien to us.

One trick they can use is to deform the shape of their paddle. By cleverly contorting the paddle create more drag on the power stroke than on the recovery stroke, single cell organisms like paramecia break the symmetry of their stroke and thus elude the scallop conundrum. Indeed, this is how the flapping structures known as cilia thrust a cell forward: they flex.

There is an even more ingenious solution that has been hit upon by bacteria, sperm and other cells. Rather than having a cilia, which is essentially a flexible paddle, these cells adopt a different strategy: they use a corkscrew for a propeller. Just as a corkscrew used on a wine bottle converts winding motion into motion along its axis, these organisms spin their helical tails (flagellum) to push themselves forward.

But don’t expect to see human swimmers doing ‘the corkscrew’ anytime soon. This strategy works only at low Reynolds number, where water ‘feels’ as thick as cork, so you can push against it effectively.

And here’s proof. Whereas our rubber band powered stiff paddle couldn’t make any headway in the corn syrup, take a look at what happens if you instead have a helical propeller.

It winds its way into the fluid and inches forwards.

Motion in this viscous world is counter-intuitive and puzzling. By applying science, we can imagine what it must feel like to be very small. And we can work out how to build tiny ships in such a world. But evolution has beaten us to the punchline, and microorganisms have evolved intricate and wonderful structures that pulsate rhythmically and take advantage of the quirks of physics at this scale.

References

Purcell, E. (1977). Life at low Reynolds number American Journal of Physics, 45 (1) DOI: 10.1119/1.10903

Avila K, Moxey D, de Lozar A, Avila M, Barkley D, & Hof B (2011). The onset of turbulence in pipe flow. Science (New York, N.Y.), 333 (6039), 192-6 PMID: 21737736

Reynolds, O. (1883). An Experimental Investigation of the Circumstances Which Determine Whether the Motion of Water Shall Be Direct or Sinuous, and of the Law of Resistance in Parallel Channels. Proceedings of the Royal Society of London, 35 (224-226), 84-99 DOI: 10.1098/rspl.1883.0018

In addition to the above papers, I learnt a lot about this subject from the following excellent book, from which many of the figures in this post are taken:
Life in moving fluids: the physical biology of flow by Steven Vogel (1996)

The theme of this post came from reading a following wonderful out-of-print book that I discovered in the basement of Strand bookstore in NYC:
On Size and Life (Scientific American Library) (1983)

Image Credits

Figures from the cited papers or from Life in moving fluids by Steven Vogel are attributed in place.

Slinky by Aaron Steele

Paddle Prints by deanspic

Cartoon of eddies was lifted from Whitewater kayaking: the ultimate guide by Ken Whiting & Kevin Varette

An airfoil wing in a wind tunnel courtesy NASA Langley Research Center

Cilia on a Paramecium courtesy Yellow Tang Moodle

Difference of beating pattern of flagellum and cilia courtesy Wikimedia Commons

Watching these bees through my camera lens, I could see something quite interesting. As they landed on the flowers, they would kick up grains of pollen that would rise up like dust. And then the bees would do something quite odd – they would fiddle with their knees. I zoomed in to see what was going on.

There’s something quite peculiar about this photograph. What’s that fleshy appendage stuck to the knees of the honeybee? It looks, to me, somewhat like a human ear. And even stranger – the bees don’t have it when they arrive on the flower. But in a few minutes this thing begins to grow, and in about 15 minutes it’s as engorged as you see in the picture.

In addition to collecting nectar from flowers, honey bees also collect pollen. And what you’re seeing in these photographs is an incredible adaptation that helps bees go about their business of collection. It’s called a pollen basket, and here is how it works.

Bees are hairy creatures, and they get covered in pollen. They rake themselves clean with combs that are built into the inner surfaces of their hind legs. Next, they move all this collected pollen to a joint between the segments of their legs – their knees. This joint functions as a pollen press, and it squeezes the pollen into handy little pellets. But these pellets need to be stored somehow. And so, here is the next adaptation. The outer surface of the hind leg is concave, and it is covered in many small hairs. It’s a basket! This is where the bees store these compressed pollen pellets, and that’s what you see in the above picture. The basket is actually transparent, and so the fleshy color in the pictures above is the color of pollen.

The weird thing about this is that the basket is open at the bottom. So why doesn’t the pollen fall out? That’s because there’s a single strong hair that prevents this from happening, which functions as the lid of the basket.

Although I couldn’t quite make out the details, watching this elaborate packing process through the zoom lens was quite mesmerizing and I was merrily snapping away. The bees didn’t seem to notice me at all, but I realized that I was getting odd looks from my neighbors, so I decided it was time to take my leave.

Young children have an uncanny ability to pick up new languages. Not only do they soak up vocabulary, they also construct new sentences of their own. This ability to use grammar is the essence of language. It’s not enough to know the meanings of words, you also have to understand the structures and rules by which words are put together.

The predominant view has been that humans are unique in this ability. But any time that we utter the words ‘uniquely human’, scientists seem to take it as a challenge to disprove this notion. And language is no exception. If you’re looking for the species that most closely matches our linguistic prowess, surprisingly, you won’t find it in the apes, the primates, or even in the mammals. You have to travel to a far more distant relative, all the way to a family of birds known as the songbirds.

The vocal life of a songbird is similar to ours in many ways. They learn songs by imitating their elders. Like human speech, these songs are passed down from one generation to the next. Songbirds are also best equipped to learn songs in their youth, and they have to practice to develop their ability. They can improvise and string together riffs into new songs, and over generations these modified songs can turn into new dialects. And like us, they come hard-wired with ‘speech-centers’ in their brain that are dedicated to language processing.

But languages are not just learned, they can also be invented. A striking example comes from the deaf community of Nicaragua in the 1970s. Back then, deaf people in Nicaragua were isolated both physically and through language. By the 1980s, the government set up schools for the deaf to teach them Spanish and how to lip-read. This turned out to be an unsuccessful endeavor. The teachers were growing increasingly frustrated as they were not getting through to the students.

However, things were quite different from the point of view of the students. For the first time, they were in contact with many other deaf people, and they started to exchange gestures that they had invented in isolation. At first the teachers thought this gesticulation was a kind of mime, but the reality was far more interesting. By getting together and pooling their ideas, these children had actually invented a new type of sign language, complete with its own grammatical structure. Here was proof that a new language could be born out of cultural isolation, a testament to our innate abilities to understand grammar. And in a few generations, users of this language were employing newer, more nuanced grammatical structures.

And this re-invention of language has been mirrored in the songbirds. An experiment from 2009 by Fehér and colleagues took newly hatched songbirds of the zebra finch species and raised them in sound proof chambers. They did this during their critical period of language development. Much like the Nicaraguan children, these birds were raised in a world without song. What happened next is quite surprising.

Just like the children, this culturally isolated generation of birds began to develop their own songs. These songs were less musical than your typical songbird song – they had irregular rhythms, they would stutter their notes, and the notes would sound more noisy. But the researchers were curious where this would lead. They listened to the songs of the next few generations of pupils, the offspring of these children of silence. What they found was quite amazing. In just two generations, the songs started to change in unexpected ways – they were becoming more musical. In fact, they started to converge upon the song of the wild songbirds, even though none of these birds had ever heard the wild songs.

I find this a rather poetic thought – these songbirds are somehow carrying within them the songs of their ancestors. This study suggests, but does not prove, that songbirds must have an innate understanding of the structures of their language. In other words, they seem to have a built-in intuition about grammar. Over time, they may be using these intuitions to develop their phrasing and tone.

And a new study by Kentaro Abe and Dai Watanabe published in today’s issue of Nature provides strong evidence for this idea. They focused on a species of songbird called Bengalese finches. The researchers wanted to understand what sorts of songs these birds consider to be similar, and what are the features that make a song sound different to them. But how do you measure what a songbird is thinking?

The way the researchers went about doing this is downright ingenious. When a songbird hears a song sung by a member of its own species, it calls out in response. The researchers would play a song to the bird many, many times. After hearing the same song being repeated over 200 times, even the most eager bird has lost interest, and their responses dwindle away. If you now play a new song, then two things can happen. The bird may find the song similar to what it has already heard, in which case it will pay it little interest. Or it may find the song to be novel, and sing more frequently in response. So by measuring how the songbird’s response changes with a new song, you can find out whether the bird can differentiate between the songs. It’s a technique that’s familiar from our everyday conversations – you can hear when your buddy is losing interest, and when they perk up to a new story.

The researchers trained the songbirds on a particular song, and then measured their responses to slightly altered songs. What they found is that the birds could notice the difference between some variants of the song, and not between others

What kind of differences were the birds latching on to? It wasn’t the notes. Although the songs were different in a few notes, the birds would not notice the difference if you changed the songs one note at a time. Rather than responding to local changes (such as a note out of place here and there), they were somehow assessing the song as a whole. Perhaps they can understand the sentence structure?

To test the hypothesis that the songbirds are responding to changes in grammar, the researchers did something quite remarkable. They taught grammar to the songbirds. They did this by inventing a set of grammatical rules, and generating 50 songs that obeyed these rules. They played these songs on repeat to the birds for an hour. Think of it as a schoolteacher drilling 50 sentences into a reluctant pupil. They then waited 5 minutes, and played the birds a new song that either fit this grammatical rule, or broke the rule. And you can guess what they found. The birds were not surprised to hear the new grammatical sentences, whereas the ungrammatical sentences would ruffle their feathers, so to speak. The birds were able to assimilate the rules of this new grammar!

“Aha!”, says the human-supremacist. “You’ve cunningly shown that these finches can learn the rules of grammar. But the grammar you’ve invented is a simple one. Human speech is far more sophisticated. For example, we can nest one sentence within another. These birds, although quite clever, surely can’t cope with such complexity.”

What our human-supremacist friend is talking about is an idea put forward in the 1950s by the linguist Noam Chomsky. He suggested that you could think of the grammar of human languages as a set of rules. By repeated use of these rules, you could go on to generate a whole lot of grammatical sentences. Starting from these rules, you could arrive at sentences like:

1. That dog laughed
2. That dog with the smelly ears laughed
3. That dog with the smelly ears that the cat disliked laughed

and so on. But you would never arrive at a sentence like:

4. That dog the smelly ears laughed

because such a sentence would violate Chomsky’s grammatical rules.

The researchers tested our embarrassing friend’s idea. Through repeated playback, they trained the finches on a set of grammatical songs, somewhat analogous to sentences 1 and 2 above. What they found is that finches with this training were not surprised by grammatical songs like sentence 3, even though these songs included an extra embedded clause. However, they would react to songs like sentence 4, that did not fit into the structure of this grammar.

Think about how incredible this is. These finches are able to understand and generalize the rules of a grammar as complex as our own.

The researchers didn’t stop there. By raising finches isolated from adults in sound-proof chambers, they were able to show that this grammatical talent is partly learned, and partly an innate ability. That is, finches that have never head the birdsong of their elders can still absorb many of these grammatical rules. And if you introduce them to their elders, over a few weeks their education in grammar will be complete. They soon become as discerning as their mentors.

What is the biological driving force behind this talent for grammar? Our brains have specific regions that ‘light up’ when we listen to a grammatically invalid construction. People who suffer damage to a specific area of the brain known as Broca’s region have an impaired ability to understand and produce grammatical speech. The authors claim to have identified regions in the finches’ brains that are necessary for their grammatical talent. By specifically removing this piece of the brain, they were able to show that the birds that went through this surgery were less competent at detecting grammatical differences.

The story of the songbirds is one of piecing together the language of the birds, and it takes us back to an ancient metaphorical quest. Many religions and mythologies have considered the language of the birds to be a symbol of great wisdom. Alchemists and practitioners of Kaballah thought it the key to perfect knowledge. Norse mythology speaks of two ravens named Huginn and Muninn (the old Norse words for thought and mind) that belonged to the god Odin. Huginn and Muninn would scour the Earth in search of news. When they returned, they would sit on Odin’s shoulders and fill him in on the affairs of mortals. I wonder what they would tell Odin about these modern-day augurs who are steadily deciphering the source of his wisdom.

References
Kentaro Abe, & Dai Watanabe (2011). Songbirds possess the spontaneous ability to discriminate syntactic rules Nature Neuroscience DOI: 10.1038/nn.2869

Fehér, O., Wang, H., Saar, S., Mitra, P., & Tchernichovski, O. (2009). De novo establishment of wild-type song culture in the zebra finch Nature, 459 (7246), 564-568 DOI: 10.1038/nature07994

To hear more about the Nicaraguan sign language, and other interesting stories about Words, check out the radiolab episode by the same name.

Image References

Opening image: an 18th centuty Icelandic manuscript depicting Odin, Huginn and Muginn. Wikimedia Commons.

Zebra Finch by Marj Kibby. Creative Commons.

Bengalese Finches image was taken from the press material provided with the paper.

That dog with the smelly ears laughed. Image and example courtesy Adam Parrish.

It feels good to be an animal. Unlike trees that are tethered to the ground, we animals have the incredible ability to travel. And we do so in a variety of ways. Some like to walk, others run. Others get around by swimming or flying. There are climbers, leapers, and hoppers, and others that prefer to roll and tumble.

Locomotion certainly affords us a great deal of freedom, but it comes at a considerable energy cost. Through countless generations of incremental evolution, our bodies have arrived at many solutions to balancing our energy budget. Fish have streamlined profiles, birds have hollow bones to stay light, and kangaroos have spring loaded hind legs that seamlessly capture and release the energy needed for flight. In the African savannah, predators chase down their prey using long, muscular legs that give them an efficient stride.

In addition to changes in form, animals can also use strategies to travel more efficiently. Birds that need to fly a long distance often make use of a curious technique. They flap their wings to gain height, and once they builds up enough height, the wings stop moving and they glide back downwards. Many birds repeat this wave-like motion in flight, instead of flying at a fixed altitude.

It’s like the difference between cycling on flat terrain or on an undulating, hilly road. In one case you pedal at a steady pace, in the other you alternately pedal hard and don’t pedal at all. The reason that birds adopt this undulating flight strategy is that it saves them energy.

But what’s special about air? What about animals that live in water? In the ocean, swimming is the equivalent of flying. So do marine animals adopt similar swimming strategies to conserve energy? To answer this question, an international group of researchers led by Adrian Gleiss attached sensors onto sharks and seals. They monitored the swimming motion of the whale shark, the white shark, the northern fur seal, and the southern elephant seal.

Here is an animation that they made directly from their recordings, that shows a whale shark swimming.

It’s as if they’re climbing an imagined hill – they work on the way up, and glide back down. In fact, all four species adopted this undulating swimming strategy. This figure, from their paper, summarizes the authors’ point.

For each animal, you see two plots. The first is a plot of its acceleration, and the second is a plot of its depth. By comparing the two, you can see that all the animals are swimming on the upslope to gain height, and then gliding back down effortlessly, just like a bird, or a cyclist on a hilly road.

The authors emphasize that this is especially remarkable, as these species have distinct evolutionary histories, and very different modes of propulsion. Elephant seals swim using hind limbs modified into flippers, fur seals use their pectoral muscles, and sharks use their tail fin. And yet, we find that in the ocean and the sky, species that are separated by millions of years of evolution are united in their solutions to one of life’s basic problems – how to get around effectively.

But there’s still a puzzle: if wavy swimming is more energy efficient, why don’t all fish do it? Why do some fish swim in this fashion but others chose to swim continuously? The authors claim that it’s all got to do with whether you naturally float or sink. They support this idea with an interesting observation: seals that swim in shallow water do so continuously, but those swimming at greater depths undulate and swim intermittently. The difference is stark – once they exceed a certain depth (15 meters, in the case of the elephant seal), they suddenly become wavy swimmers.

Here’s why the scientists think this happens. If you take a person and (very temporarily) submerge them in water, odds are that they will neither sink nor rise. That’s because humans are what is known as neutrally buoyant, meaning the density of our body exactly matches that of the surrounding water. (This is not quite true. I have a few friends who swear that they sink in water, and they’re probably right. As with any average quantity, there are some that exceed the mean, and others that don’t.)

Seals are in the same boat as us, they don’t need to work to stay afloat. They are naturally buoyant, and so swimming in the wavy way is unnecessary. However, as they dive deeper, things begin to change. As the pressure of the water increases, it squeezes their bodies into a smaller space. The same mass is now packed into a smaller volume, so the seal has become denser than water. Instead of floating, it now sinks. The authors argue that in such a situation, it makes more sense to swim wavy.

And the sharks support this idea. Unlike the seals, they don’t have lungs, or gas bladders like many other fish have. There’s nothing particularly squishy in a shark, and so their body has the same density regardless of depth. And this density exceeds that of water. This means that sharks have to swim to stay afloat. When a shark dies, it sinks like a rock.

According to the authors, this is why sharks always swim in this wavy fashion, irrespective of depth. They suggest that the more likely an animal is to sink, the more of an energy boost it gets from swimming in this interrupted manner. In this way, these heavy animals (or more accurately, dense animals) have all hit on the same clever strategy for getting the best mileage.

Reference:

Gleiss AC, Jorgensen SJ, Liebsch N, Sala JE, Norman B, Hays GC, Quintana F, Grundy E, Campagna C, Trites AW, Block BA, & Wilson RP (2011). Convergent evolution in locomotory patterns of flying and swimming animals. Nature communications, 2 PMID: 21673673

Image References:

All figures are from the paper. The youtube video is an upload of the supplementary video attached with the paper.

Opening image: There is no E.T. around, by Camil Tulcan. Creative Commons licensed.

The image of a floating woman was taken in Weeki Wachi Springs, Florida (1947) by Toni Frissell. Public Domain.

Why do we have sex? If this question keeps you up at night, you either have really loud neighbors, or you have the makings of an evolutionary biologist. Some of the most brilliant minds in the field – William Hamilton, John Maynard Smith and George Williams – have spent much of their careers wondering about the value of sex. This is not a reflection on the quality of their sex lives. Rather, it has more to do with their creative insight and ability to look at the world with fresh eyes.

A billion years ago, our ancestors inhabited a world without sex. This was the era of the clones. In this strange world, all organisms reproduced by creating identical genetic copies of themselves, somewhat similar to how modern-day bacteria reproduce [1]. But this clonal strategy has a problem. Populations made up of identical twins are more vulnerable to infection. When a disease comes along, it doesn’t just wipe out a few individuals. It can take out the whole lot.

When sex arrived, it introduced a new pace to life. Organisms were mixing and matching genes in combinations never seen before. Imagine a world where you had to dress well to survive. In such a world, the invention of sex is like going from wearing uniforms to having your own wardrobe. You could pick a gene from here, another from there, and put together a novel offspring. And if a particular outfit were deemed ‘unfit’, it’s not a huge tragedy as there are plenty of alternatives.

In this way, sex helps us by innovating new evolutionary solutions and by protecting us from disease. But sex is not without its discontents. For one thing, sexual reproduction implies that you only pass down half your genes to your offspring. The other half come from the other parent, and they combine to make an offspring with a full set of genes. On the other hand, in asexual reproduction, the mother passes on a full set of genes to her offspring. So by adopting sex, your genes are travelling half as far. In evolutionary terms, this is a huge cost, and sex had better have a lot to offer for it.

Do the benefits outweigh the costs? We would certainly like to think so. But when evolutionary biologists did the math, they worked out that the answer is usually no. Your genes typically have more to gain if you reproduced asexually.

So what gives? Why, then, do so many species adopt a sexual lifestyle? Well, here’s a brilliant solution offered by Hamilton and others: if you are under constant attack by rapidly evolving parasites, then sex is a better strategy than cloning yourself. This idea came to be known as the Red Queen hypothesis and can be summarized in one line: it’s harder to hit a moving target.

According to this theory, the main purpose that sex serves is to rapidly change our genetic makeup in order to keep pace with the threat from parasites. The parasites themselves are also evolving in order to keep attacking us. It’s a race where everyone is running but no-one really gets ahead, quite like the race between the Red Queen and Alice in Through the Looking Glass. While this does have a nice literary ring to it, I prefer a more space-age analogy. I picture the eternal chase between the evil robots and the human race in Battlestar Galactica. Technology keeps evolving on both sides, and so the humans have to work just as hard as ever to stay one step ahead.

The Red Queen idea was a theoretical offshoot of evolutionary theory. And like any good theory, it made a clear, testable prediction. Species that are exposed to a greater load of parasites should be more likely to reproduce sexually. In the last few years this idea has found support in a beautiful series of experiments involving snails, led by Kayla King, a graduate student in the lab of Curtis Lively at Indiana University.

Snails have interesting sex lives. In many species, the snail has a choice – it can either mate with another snail, or it can directly clone itself. Hedging bets by ‘going both ways’ is a remarkably common strategy in the tree of life [2].

In a study published in 2009, the researchers focused on a type of snail called the New Zealand mud snail, that inhabited two different lakes in New Zealand. Crucially, both lakes also had a parasite that infected the snails. The parasite is called microphallus, a bit harsh for a worm that’s only a fifth of a millimeter long as an adult, in my opinion. And these parasites have a strange and alarming life cycle.

Picture this: the eggs of microphallus are eaten by the snails. They hatch into a larva, which begins to grow in the snail’s gut. The larva drills through the intestine, making its way to the reproductive organs. Here the parasite begins to multiply, and consumes much of the snail’s reproductive and digestive tissue, rendering it completely sterile. Eventually the body of the snail contains hundreds of tiny cysts. When a duck comes along and eats the snail, the next stage of the parasite’s life begins. These cysts hatch to form tiny worms, which spend their entire adult lives in the duck’s intestine. Here they meet other worms, mate and produce eggs – which completes the life cycle. If this picture sends a shiver down your spine, you’re not alone.

Now, ducks only live near the lake’s surface. And the parasite can’t survive without ducks, which means that it is basically confined to shallow water. If the Red Queen idea is correct, then a heavy parasitic load should lead to intense evolution of the host, through sexual reproduction. To test this idea, the researchers went to two different lakes and compared the snails that lived in shallow waters to those found in greater depths.

Here is what they found. If you collected both shallow and deep snails and exposed them to their local parasites, the shallow water snails had consistently lower rates of infection. However, if you tried to infect snails from one lake with parasites from the other lake, the shallow water snails would fare just the same as the deep water snails.

This meant that the shallow water snails were indeed co-evolving with their local parasites. The researchers also found that the frequency of snails adopting to reproduce sexually is significantly higher in the shallow water snails as compared to their deep water relatives. These results are just what you would expect if the Red Queen hypothesis were true.

In a follow up paper this January, the authors studied the relationship between parasitic infections and genetic diversity in more detail. They looked at snails collected from 17 independent streams in New Zealand, and screened them for their genetic diversity, whether they were clonal or sexually reproducing, and whether they were infected by parasites of any type.

None of the 17 populations had done away with sex entirely. They found that as the prevalence of infection increased in a population, so did the percentage of sexually reproducing snails. As in the previous experiment, this suggests that parasitic load is driving populations to adopt sex for reproduction. They also found that among the snails that were asexual, as the prevalence of infection increased, so did the diversity between clones. What this suggests is that parasitic load is doing more than just driving sexual reproduction. It is also alleviating one of the main problems of the clonal strategy – lack of genetic diversity.

The Red Queen hypothesis is an out-of-the-box solution to a scientific conundrum. It is creative, theoretically consistent, and makes clear cut predictions – the hallmark of good science. And personally, I find it cool because it teaches us that the little guys matter. The idea that tiny microscopic life forms are driving the evolution of macroscopic beings completely topples our notions of who’s in charge here.

Footnotes

[1] Many caveats here. For one, bacteria can engage in a kind of primitive sex.

[2] All species that have switched to a completely asexual lifestyle did so fairly recently, in evolutionary terms. This must mean that many who have tried to adopt such a strategy went extinct in the long term. There is one remarkable exception, and that is the Bdelloid rotifers, who have gone without sex for 80 million years!

If you’re interested in reading more about this subject, you may be interested in:

The Red Queen: Sex and the Evolution of Human Nature by Matt Riddley

And this Nova documentary called Why Sex?

References

King KC, Delph LF, Jokela J, & Lively CM (2009). The geographic mosaic of sex and the Red Queen. Current biology : CB, 19 (17), 1438-41 PMID: 19631541

King KC, Jokela J, & Lively CM (2011). Parasites, sex, and clonal diversity in natural snail populations. Evolution; international journal of organic evolution, 65 (5), 1474-81 PMID: 21521196

Image References

Mud Snail image and Life Cycle of Microphallus from Lively lab.

Alice and the Red Queen from Through the Looking Glass (Public Domain).

Snail ecosystem figure from King et al (2009).

And death is not just the end of the individual, it’s the end of a lineage. Organisms that die before they can reproduce deny their genes a road to the next generation. In this simple fact lies the engine of change. For example, genes that make a prey more camouflaged will end up increasing their reproductive success, whereas genes that make them more noticeable are not going to get very far. In this way, natural selection is driving prey to become better hiders, and predators to become better seekers.

Nowhere is this evolutionary race more evident than in the case of the peppered moth. This is a species of moth that is found all across England and Ireland. When people first studied them in the early 1800s, almost all the moths looked something like this:

As you can see (if you’re looking closely), the white and black speckles are effective camouflage. For ages, these moths have hidden on light colored trees and lichens. Over time, they have evolved this distinctive pattern to help them evade the notice of the birds that prey on them.

But just fifty years later, things were beginning to change. By the 1850s, moths of the same species had stumbled upon a new color. These new moths were called carbonaria after their carbon-black color, to distinguish them from their salt-and-pepper colored relatives with the dull name typica.

By the end of the nineteenth century, the change was drastic. In 1895, a study in Manchester showed that 95% of the peppered moths were now of the black type. So what was going here? What could cause such an incredible change in appearance in just a hundred years?

They key lies in a major event in the history of humanity that took place during the nineteenth century – the Industrial Revolution. During this time, a large number of factories were being built in England, and they burned a mind boggling amount of coal. From 1800 to 1900, annual coal production went up in the UK from about 10 million tonnes to 250 million tonnes.

This had a drastic effect on the environment. The trees in the woods between Manchester and London were covered in soot. And the increased levels of sulphur dioxide was killing the lichen. All of a sudden, the peppered moth was losing its camouflage. It stood out like a sore thumb against the sooty black barks of the trees, while the rare black form of the moth became an instant success.

In a new study in this week’s issue of the journal Science, researchers in Liverpool and the Czech Republic were able to trace down the genetic signatures of this extreme evolution. They did this by looking at the variation in letters of DNA between the 2 types of moths.

At the heart of the idea is sex. The genetic role of sex is to shuffle together different genomes in a population. This has the effect of creating more types of genomes, and thus increases diversity.

When the Industrial Revolution comes along, it paints the world of the moth black. Most of the genomes in the population get wiped out as they are no longer fit. A few rare ones contain a gene that protects their possessor by coloring them black. These genomes quickly begin to dominate in the population, and so there are now fewer kinds of genomes around – the diversity begins to plummet (think of 1895, when 95% of these moths were now black). This is known as a selective sweep, where a set of genes rapidly sweep through a population.

Over time, as these moths mate with others, the diversity builds back. But just as it takes many shuffles to completely randomize the order of cards in a deck, it takes many generations of sexual reproduction before all trace of the past is lost in the genome. By tracing down regions of the genome with unusually low diversity, we can uncover the signals of natural selection that must have acted on our ancestors. This method of detecting natural selection works best if the selection was strong (so that it wiped out the diversity), and if it happened recently (so that sex hasn’t had enough time to bring the diversity back).

This is just what the authors did. They first compared the genomes of 68 typica and 64 carbonaria moths (the offspring of two pairs of parents) and found that a particular region on one of the chromosomes was responsible for the difference in moth color. But this is a coarse-grained picture, as the region that they identified is over a million letters in length. The next step was to probe the diversity at a finer scale.

To do this, they looked at 6 variant letters of DNA that were spread out in this region, and measured how the carboneria and typica moths vary with respect to these letters. At each of these positions, there are 2 possibile letters that any moth can have. So if the genomes were properly shuffled with the maximum level of diversity, there would be 32 64 possible possible 6 letter words that could be formed here.[1] The spotted moths were found with many different words in this region, a sign of diversity. The black moths, on the other hand, all had small variations from just one sequence: CAGGTA. The scientists inferred that this must be the ancestral sequence of the black moths that thrived in the Industrial Revolution.

By comparing our DNA, we are actually looking back in time. We can use these techniques to infer the pressures that our distant ancestors faced. A cool example of this kind of DNA archeology is the story of lactose tolerance in humans.

Here’s a counter intuitive fact – mammals typically can not digest milk in adulthood. Of course, all mammals love milk as infants (that’s what gives them their name). That’s because they can produce a chemical called lactase, which breaks down the lactose in milk. But once infants reach the age of being weaned, the body switches off production of lactase. We all like to think of cats as cute pets that love a saucer of milk, but in reality this is more likely to give them indigestion and diarrhea. Lactose intolerance is not a disease, it’s actually the norm.

This makes sense from an evolutionary point of view. Milk is a nutrient rich food for infants, but it is costly for a mother to produce. At some point, the growing infant needs to move on, or it will become too great a burden for the mother. This digestive ‘switch’ in mammals ensures that this happens.[2]

So why is it that some us can digest milk? The answer takes us from one cultural revolution to another, to a time  8000 years ago when some of our ancestors had begun to rear cattle. This was happening in the Middle East and in Africa. Through sheer chance, anyone who had a mutation that disabled this lactase switch suddenly had an advantage over their peers. They had access to a reliable and nutrient rich source of food – milk from cattle.

The same process that changed the color of the moths is at work here. As was shown by a team led by Sarah Tishkoff in 2006, cattle herders in Africa and in the Middle East independently evolved different mutations that allowed them to drink milk, an example of what is called convergent evolution [3]. This is why lactose tolerance is very prevalent in Europeans. Many of their ancestors were cattle herders who originated in the Middle East. Similarly, northern Indians are more likely to be able to digest lactose than southern Indians, perhaps due to closer contact with the pastoral Sindhi tribes of north India.

And those of use who can digest milk carry the signs of this cultural revolution in our DNA. To date, the region surrounding the lactase gene has a remarkably low diversity in populations that descended from cattle herders.

We usually think of adaptations as occurring in response to changes from within nature. But I find it fascinating that our culture can also be a driving force of evolution. It has happened time and again, without our explicit knowledge of it. During the dawn of agriculture, we evolved wild grains into harvestable varieties like wheat and rice. In the birth of pastoralism, we modified our cattle to produce more milk, while also evolving ourselves to be able to consume it. And in the Industrial Revolution, our pollutants ended up driving evolution in moths.

As we look further out into space, we learn more about the origins of our universe. But at another extreme, by looking inwards to our DNA, we are also learning more about our place in it. We are unraveling the lives and cultures of our prehistoric ancestors, as well as the effect that we have had and continue to have on our surroundings.

Incidentally, the story of the moth has another surprising twist. Eventually the air quality improved in the UK, and the lichen began to grow back. The trees were restored to their lighter colors. And this meant that the carbonera moths have once again started to get noticed. They are now at a great disadvantage and have become extremely rare. In this way, the ebb and flow of genes are echoing the waves of cultural changes.

Footnotes

The kind of evolutionary genetics discussed in this article is the subject of my research. I have spent the last year and a half working on a project that studies how certain African pastoral tribes have evolved protection to their extreme diets, a case of culture and gene flow being intricately woven together.

[1] There are 2^6 = 32 64 combinations in all. The reason for there being 2 possible letters at these variant locations and not the usual 4 (A, C, T and G) has to do with biology. The variants arise through mutations (for example, an A gets flipped to a T) in somebody. It is incredibly unlikely that two mutations in the recent past (say, A to T and A to C) will occur at exactly the same place.

[2] However, if this lactase disabling switch was only useful to the mother, than it wouldn’t evolve. But such a switch also benefits her genes, as she can now invest the resources that she gains on caring for her offspring or on rearing more children.

[3] Incidentally, the peppered moth also occurs in North America, and there are reports that a similar adaptation towards darker moths arose along with the rise in pollution in the nineteenth century. If true, than this is another neat example of convergent evolution.

References

Van’t Hof AE, Edmonds N, Dalíková M, Marec F, & Saccheri IJ (2011). Industrial Melanism in British Peppered Moths Has a Singular and Recent Mutational Origin. Science (New York, N.Y.) PMID: 21493823

Tishkoff SA, Reed FA, Ranciaro A, Voight BF, Babbitt CC, Silverman JS, Powell K, Mortensen HM, Hirbo JB, Osman M, Ibrahim M, Omar SA, Lema G, Nyambo TB, Ghori J, Bumpstead S, Pritchard JK, Wray GA, & Deloukas P (2007). Convergent adaptation of human lactase persistence in Africa and Europe. Nature genetics, 39 (1), 31-40 PMID: 17159977

Image Credits

Creative Commons Licensed: Peppered moth (typica) by Andy Phillips. Black moth (carbonia) by naturalhistoryman. Cat and milk by Sunfox. Maasai herder Kamaika Kingi by Oxfam International.

Wikimedia Commons Licensed: Light and dark moth (typica and carbonia)

Indians today can hardly recall the last time that they saw a vulture. In the 1990s, these majestic birds were a common sight in the subcontinent, and would show up wherever there was exposed carrion. As a child, I remember marveling at vultures circling at impressive heights, probably looking back down at me with their incredible eyesight, their wings outstretched as they effortlessly hovered on columns of warm air.

But since the nineties, their numbers have been falling dramatically in India, Pakistan and Nepal. The scale is astonishing – for every thousand white-rumped vultures in 1990, only one is alive today. A similarly sad story holds for the Indian vulture and the slender-billed vulture. Together, all three Asian vultures are now listed as being critically endangered.

So what’s going on? It’s not that they are being hunted. For one thing, the killing of all wild animals in banned in India. But also, vultures have always provided a much valued ecological service. Most villagers dispose of dead animals by dumping the carrion. And they rely on the vultures to clean up.

Vultures have an undeservedly bad reputation. Because we associate carrion with disease, people believed that vultures spread diseases. But in fact, we now know that the opposite is true. Their powerfully corrosive stomach acids allow them to safely digest carrion that would be lethal to other scavengers, wiping out bacteria that can cause diseases like botulism and anthrax. They are the purgers of death and disease.

In their absence, populations of feral dogs have exploded, bringing with them the threat of rabies and human attacks. And if rats follow suit, India would face a new public health nightmare as it tries to control the spread of rodent-borne diseases like bubonic plague [1].

The absence of vultures is also having a cultural impact. The Zoroastrian Parsis in India have long maintained a tradition of sky burials. They leave their dead out on platforms for the vultures to consume, in order to avoid defiling earth, water, and fire with what they consider to be an unholy corpse [1]. These towers of silence, as they are known, would once attract many hundreds of vultures. Now they are eerily empty, forcing the Parsis to find new ways to deal with their dead.

So what is causing the mysterious collapse (often literally so) of vulture populations? It’s a daunting puzzle to solve, and in 2003 an international collaboration of scientists stepped up to the challenge. Their work [2] was supported by the US-based Peregrine fund and in collaboration with the Ornithological society of Pakistan. They discovered that most the dead vultures had pasty chalk-like deposits of uric acid crystals on their internal organs. This is a terrible disease called visceral gout, and is a sign of kidney failure.

But what was causing the kidney failure?

To solve this, the authors systematically began to rule out possible explanations, in a manner that would make an episode of CSI look like child’s play. They established that it wasn’t pesticides or heavy metal poisoning, nor nutritional deficiency or a bacterial or viral infection. Instead, they found that the occurrence of kidney failure was correlated with the presence of a single chemical called diclofenac. Within a few days of consuming contaminated carrion, the vultures would fall sick, begin to droop their necks severely, and then collapse. Sometimes they would fall right out of their perches.

In essence, we were unintentionally poisoning the vultures. Diclofenac is an anti-inflammatory drug that is used by livestock farmers in India to treat their cattle and water buffaloes. Studies have since identified a vulture-safe alternative. In a last ditch move to rescue the vultures, the India’s National Board for Wildlife recommended a ban on diclofenac in 2005. A year later, this resulted in a manufacturing ban on diclofenac for veterinary use, and it was two more years before it was made an imprisonable offense to produce, sell or use this drug for veterinary purposes in 2008. All the while the vulture numbers had been falling steadily.

So how effective has this ban been in rebuilding the vulture populations? This question was addressed by another international collaboration, in a study [3] published last week. This work was led by the Royal Society for the Protection of Birds in the UK, and the researchers hailed from institutes in the UK, Spain, and from wildlife conservation societies in India. They measured the concentration of diclofenac in 4500 liver samples from 21 locations across India, taken from carcasses before and after the ban on diclofenac.

Here’s what they found. When comparing 2004 (pre-ban) to 2008 (post-ban), the percentage of samples that were contaminated went down from 10.1% to 5.6%. The concentration of diclofenac in these contaminated carcasses had also gone down, by about a factor of 2.

The next question is, what does this mean for the vultures? Is this enough of a drop in contamination for them to start making a comeback? This is a tricky question because of the limited data and the many source of errors involved. The aim of this paper was to answer it.

They combined their measurements with available numbers for how much meat the average vulture eats, and how poisonous this chemical is to them. After a careful statistical analysis, they were able to estimate the overall effect on the white-rumped vultures. What they found is that in 2004, every meal that a vulture would eat had about a 1% chance of killing it. In 2006, this reduced to a quarter of a percent chance of death, per meal. Vultures eat about every 2-3 days, so over the course of the year these percentages begin to multiply.

Finally, the researchers plugged these numbers into a simulation to work out the rate at which the vultures are dying. In 2004, their results indicated that 80% of vultures were dying every year. By 2006, about 28% to 33% of them are dying every year. So the annual death rate has gone down to more than half what it was before the ban. They extrapolate that the death rate in 2007-2008 should be about 18%. Put another way, these odds amount to every vulture having to play an annual game of Russian roulette. And these are birds that are already critically endangered.

While the drop in death rates is encouraging, the researchers remained unconvinced that enough is being done to rescue the vultures. The fact that carcasses were contaminated well after the ban points to illegal use of diclofenac. For a critically endangered population, losing more than a sixth of your numbers every year is too heavy a toll to bear. In order for the vultures to stand a chance, the government still needs to focus its efforts on a stronger enforcement of the ban, as well as take on further conservation measures in parallel.

The story of the declining vultures is yet another reminder that ecosystems are fragile, interconnected and delicately balanced. Destroying a species can affect our own health, our environment, and even our culture in ways that are near impossible to predict.

If vultures vanish from the Indian subcontinent, it would certainly adversely affect the lives of its human inhabitants. We can try to put a dollar value on what the loss would cost us. Such cost versus benefit type of calculations can make a compelling case for rescuing endangered species and maintaining biodiversity.

Yet I have always felt that they miss an important part of the picture. There is another reason that we should value the vultures, that has less to do with economics and more to do with ethics. That reason is this: in our negligence, we would be responsible for the loss of these majestic birds, as well as the 3.5 billion years of evolutionary baggage that they have carried with them. And I’m not sure that we can put a price on that.

References

[1] Gross L (2006). Switching drugs for livestock may help save critically endangered Asian vultures. PLoS biology, 4 (3) PMID: 20076536

[2] Oaks JL, Gilbert M, Virani MZ, Watson RT, Meteyer CU, Rideout BA, Shivaprasad HL, Ahmed S, Chaudhry MJ, Arshad M, Mahmood S, Ali A, & Khan AA (2004). Diclofenac residues as the cause of vulture population decline in Pakistan. Nature, 427 (6975), 630-3 PMID: 14745453

[3] Cuthbert, R., Taggart, M., Prakash, V., Saini, M., Swarup, D., Upreti, S., Mateo, R., Chakraborty, S., Deori, P., & Green, R. (2011). Effectiveness of Action in India to Reduce Exposure of Gyps Vultures to the Toxic Veterinary Drug Diclofenac PLoS ONE, 6 (5) DOI: 10.1371/journal.pone.0019069

Image Credits

The header image is of an Indian vulture, courtesy B V Madhukar. The two images of the White-rumped vulture are taken by Umang Dutt. All three images are shared under the Creative Commons License.

The image of the Parsi Tower of Silence and the Vulture distribution map are from the Wikipedia Commons.